1. Technical Field
The present disclosure pertains to ultrasound imaging and, more particularly, to a method and system for determining and utilizing image field point characteristics for image formation processing.
2. Description of the Related Art
Ultrasonic imaging utilizes high frequency sound waves to visualize the interior of objects. For example, acoustic sensing devices, such as ultrasonic inspection equipment, are used in inspecting the interiors of a variety of objects, including the human body, the area around a weld, and manufactured products such as wood-based panels. While the present disclosure is described in the context of tissue imaging, such as medical sonography, it will have application outside this field.
Medical ultrasound Imaging has developed into an effective tool for diagnosing a wide variety of disease states and conditions. The market for ultrasound equipment has seen steady growth over the years, fueled by improvements in image quality and the capability to differentiate various types of tissue. Ultrasound imaging has always required extensive signal and image processing methods, especially for array systems employing as many as 128 or more transducer elements, each with unique signal processing requirements. The last decade has seen a transition to the improved accuracy and flexibility of digital signal processing in almost all systems except for those at the lowest tiers of the market. This transition has provided the potential for improved methods of image formation that can utilize more of the information in the transmitted sound waves and returned ultrasound echo signals.
Commercial ultrasound systems typically utilize focused transmit beams for image formation. A two dimensional image field is typically insonified with a set of transmit beams that are spaced uniformly across the width of the field, each focused at a depth in the field where the best image resolution is desired. The returning ultrasound echoes from each sequential transmit beam are received and processed to obtain one of more lines of image data, where the lines correspond to the axis of each transmit beam in the set. The multiple image data lines are then interpolated into a pixel array to produce an image.
The foregoing method is illustrated in FIG. 1. A typical sequential line scan 30 might comprise 128 transmit beams 32, resulting in 128 image lines 34 that are then interpolated into pixels 36 for display. In general, the transmit beams 32 are considerably broader than the reconstructed image line 34, especially at depths other than the focal depth of the transmit beam. This results in the lateral resolution varying with depth, with the sharpest resolution obtained at the transmit focal zone as shown in FIG. 1. Only the field points along the axis of the beam are used for reconstructing the image parameters, resulting in a set of image lines 34 equal to the number of transmit beams 32 used in the scan. Because this set of image lines 34 is generally sparsely spaced relative to the spacing of pixels 36 in the display 38, the points in the image lines 34 must be interpolated for each pixel 36 in the display.
Modern commercial systems attempt to improve lateral resolution over a larger depth of field by utilizing multiple transmissions at each sequential scan position across the width of the field. At each position, the multiple transmit beams utilize different focal zones spread over the depth of interest. The image line data from each zone are combined, providing a larger effective depth of field. This technique improves lateral resolution at the cost of increased acquisition time, or lower frame rate. The time to produce a full image frame is the sum of the times needed for acquiring echo signals from each of the individual transmit beams, which is generally limited by the speed of sound and the maximum depth of interest in the medium being imaged. The more transmit beams utilized for each image frame, the longer it takes to acquire the image frame and the slower the frame rate.
Because frame rate is an important factor in many ultrasound applications, another technique is often utilized to reduce image acquisition times. The transmit beam is weakly focused so that multiple receive lines can be reconstructed in each beam, thus allowing the transmit beams to be spaced further apart over the width of the field, and reducing the total number of transmit beams needed to cover the image field. While this technique speeds up image acquisitions and can recover some of the frame rate lost to the use of multiple focal zones, lateral resolution is typically degraded due to the broadening of each transmit beam.